In the field of pressurized plants it is usual to put a first and a second compressor systems in series, for example when high final pressures have to be reached.
The term “compressor system” means—in general—both a single compressor, for example a single-stage or multistage (more particularly centrifugal) compressor, and a plurality of compressors driven by a gas turbine and/or a steam turbine and/or a variable speed electric motor and/or fixed speed motor; typically, further to said compressors (single or multistage) in a compressor system also other components are usually provided, such for example: suction/discharge isolation valves, suction/discharge pressurization valves (more particularly in parallel with isolation valves), throttling valve, suction scrubber, centrifugal compressor, after cooler, anti-surge valve, suction/discharge check valves, anti-choke valve, hot gas bypass valve, cold gas bypass valve (in parallel with anti-surge valve), blow down valve, vent valve, relief valve.
In an embodiment, in the present description, the upstream compressor system is a multistage compressor (e.g. low-medium pressure or low-medium-high pressure) and the downstream compressor system is a single-stage compressor (e.g. high pressure or very high pressure).
Typically the two compressor systems are each one operated by a dedicated driver: for example the upstream compressor system is operated by a turbine, while the downstream one is operated by an electric motor or a gas turbine; other operating means can be—in different cases—provided without departing from the present description.
The two compressor systems are connected by a pipeline where two valves are provided in parallel: an isolation valve and a pressurization valve.
While the first valve is used when the plant is running, the second one acts for the pressurization only of the downstream compressor system.
Indeed if a problem occurs in the downstream compressor system when the plant is running, it may be necessary to depressurize it (for example up to values nearly close to the atmospheric pressure) putting it in shutdown.
In some cases, this operation is accomplished by suitably venting the downstream compressor system, while the upstream one is kept running, operating in a partial or full recycle mode.
Thus it results in a situation where the upstream compressor system is in operation and it compresses the gas to a “discharge pressure in recycle mode” (or “in loop mode”) that is usually less than the “discharge pressure in operating mode”, while the downstream compressor system is at a lower pressure for example equal about to the atmospheric pressure.
When the whole plant has to be restarted it is firstly necessary to pressurize again the downstream compressor system.
This is usually accomplished by opening said pressurization valve on the pipeline connecting the two compressor systems; however the opening of the pressurization valve leads to a substantially isoenthalpic expansion of the gas with a relevant temperature drop due to the Joule-Thompson effect.
In case of a gas having a high molecular weight, the Joule-Thompson effect is particularly important and it leads to a considerable temperature drop in the downstream compressor system.
For example in the “Oil & Gas” field, such plants are used for compressing gas mixtures rich in methane and other hydrocarbons up to pressures higher than 250 bar, in order to perform gas injections in gas wells or similar; particularly it can be considered (as a non-limiting example) that a gas has a relatively high molecular weight when it is equal to or higher than a value comprised between 28-44 Kmol/Kg, depending on the gas composition; it must be stressed that this range of molecular weight is not intended as limiting the invention, but only as an indication for a range in which the advantage achieved by the invention are more strongly felt.
In these cases, during the pressurization of the downstream compressor system by means of the pressurization valve, the pressure drop at the pressurization valve is very high: for example if the upstream compressor system is a low/medium pressure multistage one, such pressure drop can reach about 250 bar; this value is exceeded if the upstream compressor system is a low/medium/high pressure multistage one.
Under such hypotheses the Joule-Thompson effect leads to a temperature drop in the gas and therefore in the downstream compressor system, such to bring the latter to very low temperatures; if the minimum temperature drops below the minimum equipment design temperature, risk of mechanical integrity issues may be achieved.
With particular reference to FIG. 1 attached, the Joule-Thompson effect depends on the type of gas and on the temperature and pressure of the gas before expansion; the Joule-Thompson effect describes the temperature change of a gas when it is forced through a valve while kept insulated so that no heat is exchanged with the environment.
Experimentally is calculated as:
      μ    JT    =            lim                        Δ          ⁢                                          ⁢          p                ->        0              ⁢                            T          2                -                  T          1                                      p          2                -                  p          1                    Where: T1 is the temperature at the valve outlet, which can be measured downstream of the valve. For example, in the pipe between the valve outlet and the compressor inlet. T2 is the temperature at the valve inlet, which can be measured upstream of the valve. For example in the pipe between the valve inlet and the compressor outlet. p1 is the pressure at the valve outlet, which can be measured downstream of the valve. For example, in the pipe between the valve outlet and the compressor inlet. p2 is the pressure at the valve inlet, which can be measured upstream of the valve.
For example in the pipe between the valve inlet and the compressor outlet. Δp is the pressure drop across the valve, which can be measured with a dedicated pressure drop devices. μJT is the Joule-Thomson coefficient; this value is not measured but calculated. which becomes, taking into account that the process is isenthalpic:
      μ    JT    =            (                        ∂          T                          ∂          p                    )        H  
The equation can be put in the differential format:dT=μJTdp at H costant
The above equations show the higher is the pressure drop, the higher the temperature drop is if the Joule Thomson coefficient is greater than zero.
As an example think that with a pressure change of 250 bar (downstream and upstream of the pressurization valve) and with an output gas temperature of about 100° C. from the upstream compressor system, with a gas having a molecular weight of about 39 Kmol/kg, the downstream compressor system has a temperature of about −54° C.
Since the Joule-Thompson effect is directly proportional to the pressure drop, in the prior art, in order to reduce the temperature drop (consequence of the pressure drop when the Joule Thomson coefficient is greater than zero) in the downstream compressor system the delivery pressure of the upstream compressor system is reduced.
This is accomplished by partially or totally venting the upstream compressor system, reducing the delivery pressure thereof, thus reducing the pressure drop at the pressurization valve and therefore, finally, by mitigating the temperature decrease of the gas and accordingly of the downstream compressor system.
Although in principle this pressurization method is functional, it has some drawbacks.
Firstly a loss of production occurs, since after reducing the delivery pressure of the upstream compressor system, it has to be brought again to the working pressure, which is time consuming and it requires some additional energy.
Secondly, in some cases, it happens that the upstream compressor system has to be completely restarted, with a further time and energy waste and a reduced production.
Thirdly, environmental impacts, due to the gas emissions, could lead to penalties.